This invention relates to a weigh feeding apparatus for pourable substances such as granulates, powders and fluids. It consists of a container equipped with a controllable device for discharging the substance, apparatus for weighing the container or a conveyor arranged subsequently to the discharging device as well as an electronic circuitry supplied with a computer for regulating and controlling the discharging device; this circuitry counts the digital measuring signals of the weighing apparatus during a period of time, compares them with a reference signal and forms a control value for the discharging device at a divergence of the signal.
There are loss-in-weight feeders used for the feeding of pourable substances on which the speed of the weigh feeder is controlled by comparing an actual value determined by weighing with a reference value. The determination of the actual value may be effected either by using the loss of weight during the discharge of a container filled with the substances or by determining the weight of a conveyor loaded with the substance. In order to achieve a good controlling accuracy it is necessary to have the controlling action at a very small deviation of the measuring signals. Unfortunately there are sometimes caused considerable disturbances due to vibrations, shocks etc by external forces acting on the weighing apparatus which could cause serious errors in the weigh feeding process.
Besides an analog processing of the measuring signals it is also possible to process them in a digital manner. In this way a digital signal proportional to the weight is produced. This can either be done by converting an analog signal into a digital signal or by using a digital weighing apparatus in which the frequency changes are measured. In the device known from DE-OS 2754527 the measuring signals and pulse signals are stored by means of gate circuits. The number of the measuring signals stored during a predetermined time period is compared with a preset number.
To monitor the controlling action the slope angle of the measuring phases is determined and then compared with the slope angles determined before. If on comparison more than a fixed number of measuring points fall off the discharging device will continue with the speed already determined and stored. Control is therefore blocked. Only if the deviations of successive measuring periods of the following measurements are within the predetermined limits control is activated again. In order to eliminate external disturbances and influences a correction factor is determined in the configuration known from DE-OS 2754527 during an operation phase when the substance is held back on the weighing device. Oscillations of the feeder drive are determined and then stored in the computer as digitized values. These will then be deducted from the measuring signals determined at the same time. When using the optional volumetric mode the controlling circuit is also interrupted and the gating values stored before for the adjustment of the weigh feeding apparatus are utilized. To ensure that the weighing and feeding will continue when filling the container of the weigh device operation during the filling is switched from the controlled gravimetric mode to the volumetric mode which is only adjustable.
In the weigh feeding devices known there is the disadvantage that the sensitivity of response cannot be adjusted sufficiently precisely due to the disturbances to be considered. Any elimination of disturbances affects the measuring accuracy as a rule. Furthermore another disadvantage is that on any interference due to vibration, shock etc. on the weighing device the controlling circuit is interrupted and is then continued as a fixed value so that a volumetric feeding mode is used which, however, does not permit an exact weigh feeding action.